Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. For example, Spinal Cord Stimulation (SCS) techniques, which directly stimulate the spinal cord tissue of the patient, have long been accepted as a therapeutic modality for the treatment of chronic neuropathic pain syndromes, and the application of spinal cord stimulation has expanded to include additional applications, such as angina pectoralis, peripheral vascular disease, and incontinence, among others. Spinal cord stimulation is also a promising option for patients suffering from motor disorders, such as Parkinson's Disease, Dystonia and essential tremor.
SCS systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the neurostimulation lead(s) or indirectly to the neurostimulation lead(s) via a lead extension.
Electrical stimulation energy may be delivered from the IPG to the electrodes in the form of an electrical pulsed waveform. Thus, electrical pulses can be delivered from the IPG to the neurostimulation leads to stimulate the spinal cord tissue and provide the desired efficacious therapy to the patient. The configuration of electrodes used to deliver electrical pulses to the targeted spinal cord tissue constitutes an electrode configuration, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode configuration represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, pulse width, and rate (or frequency) of the electrical pulses provided through the electrode array. Each electrode configuration, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”
The SCS system may further comprise a handheld patient programmer in the form of a remote control (RC) to remotely instruct the IPG to generate electrical stimulation pulses in accordance with selected stimulation parameters. Typically, the stimulation parameters programmed into the IPG can be adjusted by manipulating controls on the RC to modify the electrical stimulation provided by the IPG system to the patient. Thus, in accordance with the stimulation parameters programmed by the RC, electrical pulses can be delivered from the IPG to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. The best stimulus parameter set will typically be one that delivers stimulation energy to the volume of tissue that must be stimulated in order to provide the therapeutic benefit (e.g., treatment of pain), while minimizing the volume of non-target tissue that is stimulated.
However, the number of electrodes available combined with the ability to generate a variety of complex electrical pulses, presents a huge selection of stimulation parameter sets to the clinician or patient. For example, if the SCS system to be programmed has an array of sixteen electrodes, millions of stimulation parameter sets may be available for programming into the SCS system. Today, SCS systems may have up to thirty-two electrodes, thereby exponentially increasing the number of stimulation parameters sets available for programming.
To facilitate such selection, the clinician generally programs the IPG through a computerized programming system; for example, a clinician's programmer (CP). The CP can be a self-contained hardware/software system, or can be defined predominantly by software running on a standard personal computer (PC). The CP may actively control the characteristics of the electrical stimulation generated by the IPG to allow the optimum stimulation parameters to be determined based on patient feedback or other means and to subsequently program the IPG with the optimum stimulation parameter sets.
For example, in order to achieve an effective result from conventional SCS, the lead or leads must be placed in a location, such that the electrical stimulation energy creates a sensation known as paresthesia, which can be characterized as an alternative sensation that replaces the pain signals sensed by the patient. The paresthesia induced by the stimulation and perceived by the patient should be located in approximately the same place in the patient's body as the pain that is the target of treatment. If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy. When leads are implanted within the patient, the CP, in the context of an operating room (OR) mapping procedure, may be used to instruct the IPG to apply electrical stimulation to test placement of the leads and/or electrodes, thereby assuring that the leads and/or electrodes are implanted in effective locations within the patient.
Once the leads are correctly positioned, a fitting procedure, which may be referred to as a navigation session, may be performed using the CP to program the RC, and if applicable the IPG, with a set of stimulation parameters that best addresses the painful site. Thus, the navigation session may be used to pinpoint the VOA or areas correlating to the pain. Such programming ability is particularly advantageous for targeting the tissue during implantation, or after implantation should the leads gradually or unexpectedly move that would otherwise relocate the stimulation energy away from the target site. By reprogramming the IPG (typically by independently varying the stimulation energy on the electrodes), the VOA can often be moved back to the effective pain site without having to re-operate on the patient in order to reposition the lead and its electrode array. When adjusting the VOA relative to the tissue, it is desirable to make small changes in the proportions of current, so that changes in the spatial recruitment of nerve fibers will be perceived by the patient as being smooth and continuous and to have incremental targeting capability.
Conventional SCS programming has as its therapeutic goal maximal stimulation (i.e., recruitment) of dorsal column (DC) nerve fibers that run in the white matter along the longitudinal axis of the spinal cord and minimal stimulation of other fibers that run perpendicular to the longitudinal axis of the spinal cord (dorsal root (DR) nerve fibers, predominantly), as illustrated in FIG. 1. The white matter of the dorsal column includes mostly large myelinated axons that form afferent fibers. Thus, conventionally, the large sensory afferents of the DC nerve fibers have been targeted for stimulation at an amplitude that provides pain relief.
While the full mechanisms are pain relief are not well understood, it is believed that the perception of pain signals is inhibited via the gate control theory of pain, which suggests that enhanced activity of innocuous touch or pressure afferents via electrical stimulation creates interneuronal activity within the dorsal horn (DH) of the spinal cord that releases inhibitory neurotransmitters (Gamma-Aminobutyric Acid (GABA), glycine), which in turn, reduces the hypersensitivity of wide dynamic range (WDR) sensory neurons to noxious afferent input of pain signals traveling from the dorsal root (DR) neural fibers that innervate the pain region of the patient, as well as treating general WDR ectopy. Consequently, stimulation electrodes are typically implanted within the dorsal epidural space to provide stimulation to the DC nerve fibers.
As illustrated in FIG. 1, the DH can be characterized as central “butterfly” shaped central area of gray matter (neuronal cell bodies) substantially surrounded by an ellipse-shaped outer area of white matter (myelinated axons). The DH is the dorsal portion of the “butterfly” shaped central area of gray matter, which includes neuronal cell terminals, neuronal cell bodies, dendrites, and axons.
Activation of large sensory fibers also typically creates the paresthesia sensation that often accompanies SCS therapy. Although alternative or artifactual sensations, such as paresthesia, are usually tolerated relative to the sensation of pain, patients sometimes report these sensations to be uncomfortable, and therefore, they can be considered an adverse side-effect to neuromodulation therapy in some cases.
It has been shown that the neuronal elements (e.g., neurons, dendrites, axons, cell bodies, and neuronal cell terminals) in the DH can be preferentially stimulated over the DC neuronal elements by minimizing the longitudinal gradient of an electrical field generated by a neurostimulation lead along the DC, thereby providing therapy in the form of pain relief without creating the sensation of paresthesia. Such a technique is described in U.S. Provisional Patent Application Ser. No. 61/911,728, entitled “Systems and Methods for Delivering Therapy to the Dorsal Horn of a Patient,” which is expressly incorporated herein by reference.
This technique relies, at least partially on the natural phenomenon that DH fibers and DC fibers have different responses to electrical stimulation. The strength of stimulation (i.e., depolarizing or hyperpolarizing) of the DC fibers and neurons is described by the so-called “activating function” ∂2V/∂x2 which is proportional to the second-order spatial derivative of the voltage along the longitudinal axis of the spine. This is partially because the large myelinated axons in DC are primarily aligned longitudinally along the spine. On the other hand, the likelihood of generating action potentials in DH fibers and neurons is described by the “activating function” ∂V/∂x (otherwise known as the electric field). The DH “activating function” is proportional not to the second-order derivative, but to the first-order derivative of the voltage along the fiber axis. Accordingly, distance from the electrical field locus affects the DH “activating function” less than it affects the DC “activating function.”
While fibers in the DC run in an axial direction, the neuronal elements in the dorsal horn are oriented in many directions, including perpendicular to the longitudinal axis of the spinal cord. However, as illustrated in FIG. 2, the dorsal horn stimulation technique described in U.S. Provisional Patent Application Ser. No. 61/911,728, generates an electrical field that is uniformly in one direction. There, thus, remains a need for an improved technique to stimulate the neuronal elements of the dorsal horn.